Water treatment using an electrolytic reaction is widely performed, for example, in order to perform production of functional water, ozone water, and electrolyzed water, sterilization, decomposition and removal of a harmful substance through electrolysis. A reaction tank used in the above process generally has a structure in which an anode, a cathode, and an ion exchange membrane or a porous separation membrane which is interposed between the anode and the cathode are accommodated in a housing. Such a reaction tank is referred to as an electrolytic bath or an electrolysis cell. This type of electrolytic bath or electrolysis cell is configured by a separation membrane, an anode chamber formed by being separated by the separation membrane, a cathode chamber formed by being separated by the separation membrane, an anode provided in the anode chamber, and a cathode provided in the cathode chamber. As this type thereof, a two-chamber type electrolysis device or a three-chamber type electrolysis device is known.
As the two-chamber type electrolysis device, there are a diaphragm process type electrolysis device, a cation exchange membrane type electrolysis device, and a solid polymer electrolyte type electrolysis device which is a special type.
The diaphragm process type electrolysis device uses a porous membrane as a separation membrane. The cation exchange membrane type electrolysis device uses a cation exchange membrane as the separation membrane. The solid polymer electrolyte type electrolysis device configures an electrolysis device in which the anode and the cathode are adhered to both surfaces of the cation exchange membrane, and thus it is possible to perform electrolysis of pure water having small electric conductivity, by using the cation exchange membrane as a solid polymer electrolyte. As the three-chamber type electrolysis device, a device in which a cation exchange membrane and an anion exchange membrane as a separation membrane configured to separate an anode chamber and a cathode chamber from each other are provided between the anode chamber and the cathode chamber, and an intermediate chamber is formed between the cation exchange membrane and the anion exchange membrane is employed. In the electrolysis devices, various types of functional water and ozone water are generated.
Generally, in a waste liquid treatment process or a producing process of functional water such as so-called alkali ion water, unpurified water containing ions of alkaline earth metal, such as a calcium ion and a magnesium ion is used as a raw material. In electrolysis using such unpurified water, while electrolysis is in progress, firstly, pH of a catholyte is increased on the surface of the cathode, and thus ions of alkaline earth metal in which a small amount of calcium in raw water is the main component are deposited on the surface of the cathode, as non-conductive scale, that is, hydroxide, oxide, and carbonate thereof. As a result, continuing electrolysis often becomes difficult.
Thus, in PTL 1 and PTL 2, a method of using acid as a cathode chamber liquid is proposed. However, the configurations in PTL 1 and PTL 2 are complicated, and safety management in operation is burdened. In PTL 3, a method in which an auxiliary tank and a plurality of electrode sets are installed in an apparatus for producing electrolyzed water, and are switched and used for each predetermined time, and thus deposit in a cathode is suppressed is proposed. However, this method causes the size and cost of the apparatus to be increased. Further, in PTL 4, a method in which an operation is suspended for each predetermined time and sediment is removed by acid washing and the like is described in detail. However, the work is complicated. In PTL 5, a method in which an electrolysis cell which does not include a separation membrane is caused to have acidity by using hydrochloric acid, and thus deposit in a cathode is prevented is proposed. However, because a strongly-acidic chemical liquid such as hydrochloric acid is used, this method may be disadvantageous in an aspect of securing of safety or cost, and using of strong acid may be not accepted in accordance with the purpose of the use.
In PTL 6, a method in which an anode and a cathode of an electrolysis cell are reversed to each other when electrolytic properties are deteriorated, and recovery of performance is achieved by passing through a reverse current is proposed. In this case, when such a reverse current flows, the cathode temporarily acts as the anode, and thus the metal constituent is eluted. Many of ions themselves of the eluted metal are not preferable as ions contained in a treatment liquid for each of Cr, Ni, and the like. In addition, the ions are permeated into a solid polymer electrolyte membrane, and thus ion transfer capability thereof is significantly degraded. For this reason, valve metal having high corrosion resistance may be used in the cathode. However, in this case, expensive precious metal coating and the like may be performed on the surface of the valve metal, and, if the coating is not performed, lowering very high electrolysis overvoltage is required. In addition, deterioration of an electrode catalyst or an electrode base by cathodic reduction of the anode which temporarily functions as the cathode, or hydrogen embrittlement occurring by cathodic reduction is also concerned.
Further, according to PTL 7, a method of producing hypochlorite, in which electrolysis of a chloride aqueous solution is performed without separation membrane by using a cathode in which a coated film which is formed on a conductive base and has low hydrogen overvoltage is covered with a reduction prevention coated film, is proposed. As the reduction prevention coated film, an organic cation exchange membrane body, an inorganic cation exchange membrane body, or a mixture thereof is used. However, in an electrolysis method performed without a separation membrane, that is, a method in which a matter generated on the anode is directly brought into contact with the cathode, the reduction prevention coated film functions to prevent reduction of ions of hypochlorous acid, which occurs by the cathode, but does not function to prevent precipitation of cathode deposition which is mainly formed of hydroxide of alkaline earth metal, on the cathode. In an electrolysis method and an electrolysis device using a separation membrane as in the present invention, a reduction prevention film for preventing reduction of ions of hypochlorous acid which is a product in an anode, as described in PTL 7, is not required.
In the electrolysis method and the electrolysis device using a separation membrane in the related art, in a case where unpurified water containing ions of alkaline earth metal is used as a raw material, metal ions ionized as cations is concentrated on the surface of a cathode, and pH is increased by OH− ions generated on the cathode. As a result, scale which is mainly formed of hydroxide precipitates as a cathode deposition. Operation inhibition by the formation of the scale causes a problem. However, in a method of suppressing formation of scale, which has been conventionally proposed, a negative aspect in that corresponding cost and labors are required, or a portion of capability is to be abandoned is large. Thus, improvement is desired.
Ozone water exhibits an advantageous effect of sterilization and the like when ozone in the ozone water is decomposed. However, after the ozone is decomposed, only stable oxygen remains. Thus, the ozone water attracts attentions as a treatment agent having a very low environmental load. Currently, the ozone water is used for decomposing an organic matter, for example, used for sterilization or decolorization, deodorization, or the like. Henceforth, further wide use of the ozone water, for example, for preventing infection diseases is expected.
In an ozone water generation cell by an electrolysis process, generally, a so-called membrane-electrode assembly is configured as a function unit. The membrane-electrode assembly has a structure in which an anode for generating ozone, such as a diamond electrode, a cathode formed of stainless steel or the like, and a cation exchange membrane interposed between the anode and the cathode are strongly adhered to each other. If a direct current is applied between the anode and the cathode in the membrane-electrode assembly, oxygen and ozone are generated on the surface of the anode in a form of a gas, and a considerable amount of the ozone gas is dissolved in the surrounding raw water. Thus, water in which an ozone gas is dissolved, that is ozone water, is generated.
One problem in the above process is that generation efficiency of ozone water is much lower than a theoretical value for ozone gas generation.
The inventors found that the amount of dissolved ozone gas, which determines ozone water generation efficiency, strongly depends on a flow rate of raw water in the vicinity of the electrode. However, it is considered that this phenomenon suggests the followings: a point that local ozone concentration in water is rapidly locally saturated in the ozone evolution site; and a point that fine ozone gas bubbles just after evolution stay in a gas generation site, and rapidly grow to be larger gas bubbles, and as a result, it becomes difficult for the ozone gas to efficiently dissolve.
Considering the above problems and afterward marketability, the inventors proposed an electrolysis cell which was to solve the above problems, and had a structure in which a plurality of through hole were provided in a membrane-electrode assembly, and raw water passed through the holes in unidirection. The inventors applied for a patent (PTL 8) which disclosed that it is possible to improve ozone water generation efficiency by the proposed electrolysis cell.
According to an apparatus for producing electrolyzed ozone water in PTL 8, an anolyte (acidic ozone water which is an anode product) in an anode chamber and a catholyte (alkali hydroxide which is a cathode product) generated in a cathode chamber are mixed, and integrally flow out. Thus, the catholyte is mixed with the acidic ozone water which is the anolyte generated on the anode, and pH on the surface of the cathode is lowered from alkalinity to the vicinity of neutrality. Thus, an occurrence of a situation in which scale which is mainly formed of hydroxide of alkaline earth metal precipitates on the surface of the cathode is considerably suppressed.
However, the followings are understood in the apparatus for producing electrolyzed ozone water in PTL 8. That is, a rigid material such as precious metal, nickel, stainless steel, and titanium is used in the cathode. Almost all of scale which is mainly formed of hydroxide of alkaline earth metal, such as calcium hydroxide and magnesium hydroxide, which precipitates in the vicinity of a contact interface between the cathode and a solid polymer electrolyte separation membrane without coming into contact with acidic ozone water which is an anolyte generated on the anode side is not stored in the cathode, and does not pass through multiple through holes formed in the cathode. Almost all of the scale is deposited at the contact interface between the cathode and the solid polymer electrolyte separation membrane. Thus, continuing electrolysis may be interfered.
In addition, the followings are understood. The cathode is formed from a rigid material, and does not have flexibility. Thus, even though the cell is formed as a solid polymer type electrolysis cell by compressing from both sides thereof, the cathode, the anode, and the solid polymer separation membrane are not sufficiently adhered to each other, and a cell voltage is increased.